This invention relates to a circuit design and associated method for implementing current-based analog dimming of Light Emitting Diode (LED) illuminators, and in particular to a circuit design and method that improves the performance and accuracy of the dimming function, at low levels of current through the LED devices.
One of the basic required functions of the control circuits for LED illuminators is the ability to control the dimming or brightness of the LEDs. In most LED illuminator designs, direct current (DC) is fed through individual LED devices, or strings of LED devices, causing them to emit light. There are two basic methods of controlling the brightness of the LED devices, or dimming them. One of these basic methods is to vary the level of the direct current that is fed through the LED devices, with the brightness level of the LED illuminator being roughly proportional to the level of current. This method will henceforth be referred to as either current-based dimming, or analog dimming. The other basic method is to use a fixed current amplitude, and then to interrupt the flow of current at some frequency and duty cycle. This latter method is typically referred to as Pulse-Width Modulation (PWM) dimming. Because the human eye can integrate or average the pulses of emitted light, the perceived brightness of an LED illuminator that uses PWM dimming is basically proportional to the duty cycle of the pulsed LED current.
The two basic methods of dimming have different advantages and disadvantages. Generally speaking, PWM dimming is viewed as providing well-controlled and repeatable dimming, since the perceived brightness is tightly correlated with the duty cycle of the PWM signal, and it is fairly straightforward to generate a consistent and repeatable PWM signal. The primary disadvantage of PWM dimming is due to its very nature, and is related to the fact that the LEDs are being turned rapidly on and off. Although this is usually not a problem for human vision, it can cause problems with photography, videography, and some machine-vision applications that require the light source or illuminator to be ON at all times, without pulsing.
Although current-based analog dimming solves the fundamental issues associated with PWM dimming, by virtue of providing a constant current to the LEDs (i.e., without pulsing), there are other problems associated with this method. The most fundamental issue is that the light output of LED devices is only approximately proportional to the current flowing in them, with diminishing efficiency as the current increases, and as the junction temperature of the LED devices increases. However, it is possible to compensate for this non-linear behavior by calibrating the level of current provided, for different intended brightness levels. Other sources of inaccuracy in the current-controlled, analog dimming method are introduced by the use of commercially-available LED driver control chips or integrated circuits (ICs). The present invention provides a circuit and method that addresses one of the common limitations of commercially-available LED driver ICs, by offering current-controlled analog dimming with improved performance at low current levels and low brightness levels.
Commercially-available prior art LED driver ICs typically provide a regulated constant-current feed to one or more LEDs, that are typically connected as a series string to ensure that the same current is flowing in all of the LEDs. Current regulation is provided through the use of a low-value current-sensing resistor, wired in series with the LED or LED string. The small voltage drop across this resistor is fed back to the LED driver IC, as a representation of the current flowing through the LED(s), and the LED driver IC uses this signal to regulate the current being sourced to the LED(s).
Most such prior art LED driver ICs provide for both PWM dimming, and current-controlled analog dimming. A typical method for providing current-controlled analog dimming is to provide an input pin on the LED driver IC, to which a small control voltage is applied, such that the resulting regulated LED current will be proportional to the applied control voltage. Typically, the allowed range of control voltages that can be applied to this pin is quite small, falling well within the range of 0 to 5 volts, and more typically between 0 and 2 volts. This is so the LED driver IC can be powered by a low voltage power supply, and also so that the control voltage can be generated by a low voltage control circuit. In a typical prior art LED driver IC, a control voltage of approximately 0.2 volts (or less) will result in a minimum LED current, ideally 0 mA, and a control voltage that is greater than or equal to approximately 1.2 volts will result in maximum LED current. Control voltages between 0.2 volts and 1.2 volts result in a proportional, or linearly-scaled LED current. The exact range of intended control voltage will depend, of course, on the specific LED driver IC that is selected. The control voltage itself may be generated and controlled in a variety of ways, including the use of potentiometer or other resistive voltage divider circuit, or by a processor sending digital codes to a commercially-available Digital-Analog Converter (DAC) device. It should also be noted that some commercially-available LED driver ICs allow the user to feed a digital PWM signal into the LED driver IC as a control input, and have the capability of internally interpreting the PWM signal as an analog dimming control input, thereby effectively “converting” a PWM dimming signal to current-based, analog dimming.
Commercially-available prior art LED driver ICs typically provide fairly accurate LED current, as a function of the control voltage input, for LED currents that range from 100% of the designed maximum LED current, down to approximately 5% or 10% of the designed maximum current. However, at low LED current levels, that are less than approximately 5% or 10% of the designed maximum LED current, many LED driver ICs experience difficulty in properly regulating the current value. This manifests itself as either an inability to fully turn the LEDs off, or, alternatively, as an inability to dim fully, thereby preventing reliable achievement of low brightness levels. In the latter case, the symptom is that the LEDs will simply turn off when the selected current level is less than 5% or even 10% of the maximum current.